In a typical power system, a distribution substation receives power at a high voltage (e.g., 230 kV), and the power is transformed onto separate three-phase distribution lines at a lower voltage. The distribution lines carry the power to consumer locations, where local transformers provide service at 120 volts (or other NEMA standard voltages) to customers.
In order to meet regulatory requirements and to avoid impaired service to customers, the transformer output voltage is tightly controlled. To change a transformer's output to respond to varying customer demand, the most common method is to increase or decrease the number of active turns in one of the transformer's windings with respect to another winding (turns ratio). The process of changing the turns ratio is referred to as a tap change, and is effective to change the voltage ratio of the transformer. To maintain service continuity, the tap changes are carried out while the transformer is under load. A large variety of control systems have been developed to automate the process of tap changes under load.
In the power distribution substation, it is common to employ multiple three-phase transformers, operating in parallel. If the transformers have different turn ratios, reactive power (VARs) will circulate from the transformer with the highest open circuit output voltage to the others. As the difference between the tapping positions of the transformers increases, so does the magnitude of the circulating VARs. It is therefore desirable to employ control methods for the transformers' tap positions which consider all of the parallel transformers.
Several methods have been devised to control tap changing under load, both for systems having a single transformer, and for parallel systems. U.S. Pat. No. 4,695,737 to Rabon, et al discusses a load compensating power distribution regulator system for AC power distribution. The system includes multiple regulators and has sensors for measuring the regulator voltage and output current under computer control. The actual circuit voltage and change in output current are determined based on the measured data. Voltage regulator changes are limited to maintain the actual circuit voltage within a desired range.
The methods disclosed in the prior art rely on the transformer low voltage side bus voltage and the transformer current to determine the best tap position, when implementing a change. This method works well as long as the high side and low side of the transformers are connected. If the high side is split, as may occur during breaker maintenance, the tap changer control system may lockout (i.e., no tap changes will be permitted) if an excessive circulating current is detected.
Additional difficulties have been encountered in prior art systems which measured the circulating current between transformers. These systems tend to respond to both real power and reactive power flows, and may experience errors when the circulating real power is high.